Catalytic oxydehydrogenation process

ABSTRACT

Isobutyric acid or a functional equivalent, e.g., a lower alkyl ester is oxidatively dehydrogenated to effect the vapor phase conversion thereof to the corresponding α,β-ethylenically unsaturated derivative by contact with a heterogeneous catalyst in the presence of molecular oxygen. The catalyst is composed of calcined phosphates of iron containing silver as a modifier or dopant component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the conversion of isobutyricacid to methacrylic acid including the like conversion of a lower alkylester of isobutyric acid.

2. Description of the Prior Art

There is considerable prior art directed to the oxydehydrogenation ofthe lower saturated mono-carboxylic acids to prepare the correspondingα,β-ethylenically unsaturated acids. The initial work reported in thisarea was that of thermally effecting the indicated oxydehydrogenation bythe vapor phase reaction of the acid substrate with iodine and oxygen.This approach has not attracted much attention as a potentially viableway for commercially implementing the underlying reaction. This isunderstandably so inasmuch as iodine is costly, exhibits extremecorrosivity properties and poses considerable problems in realizingcomplete recovery of the comparatively large amounts thereof required inthe process.

As the subsequent prior art picture amply points up, the heterogeneouscatalytic method for effecting the oxydehydrogenation reaction is viewedas being much more attractive from the standpoint of potentialcommercial applicability. In the main, the more recent relevant priorart activities have centered on the use of two types of catalystcompositions for this purpose. One type includes generally theheteropoly-acids, typically representative of which 12-molybdophosphateoptionally including vanadium and/or tungsten elements in a likestructural arrangement. The other type catalyst includes those systemshaving in common a calcined iron phosphate matrix.

Iron phosphate subjected to calcination exists in a plurality ofcrystalline phases or species. While it is believed that theoxidation/reduction couple involved in the underlying reaction isattributable to the iron phosphate, which species is or arecatalytically active has not been identified. There is, however,evidence that the presence of an extrinsic metal component in thepreparation serves to facilitate the formation of the catalyticallyactive species. For example, U.S. Pat. No. 3,948,959 notably teachesthat an alkali or alkaline earth metal as the extrinsic metal componentis effective for this purpose. The present invention accordinglyrepresents a furtherance of this particular aspect of the current stateof the art.

SUMMARY OF THE INVENTION

In accordance with this invention, a catalytic process is provided foreffecting the oxidative dehydrogenation of isobutyric acid or a loweralkyl ester thereof to form the corresponding α,β-ethylenicallyunsaturated derivative. The process of this invention comprisescontacting a heterogeneous catalyst at a temperature from 300°-500° C.with a co-feed of said substrate and diluted oxygen gas, characterizedin that said catalyst is calcined iron phosphate containing theextrinsic metal silver as the modifier or dopant component. In thebroadest aspect of the invention the contemplated catalyst is defined bythe gram-atom empirical formula FeP₁₋₂ Ag₀.01-1 O_(x) in which xrepresents the number of oxygen atoms bound to the other metals in theirrespective states of oxidation in the catalyst.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

There are a number of techniques applicable for preparing the catalystuseful in the practice of this invention. Of these, the more facilemethods involve preparing the integral composition prior to calcination.This can be readily accomplished by employing the so-called slurrymethod or the precipitation method. In the latter method an aqueoussolution of salts of the contemplated metals and phosphoric acid isfirst prepared and thereupon neutralized with an appropriate base inorder to precipitate the mixed metal phosphates. The precipitate isdesirably carefully washed to remove all traces of water solubles andthen dried prior to calcining. In the alternative, one can add ammoniumphosphate to the solution of metal salts in order to precipitatedirectly the metal phosphates. As indicated, any water-soluble salt ofiron or silver can be used. However, because of the solubilitycharacteristics of the nitrate salts, among other reasons, such saltsare preferred.

The so-called slurry method is even more convenient to carry out and forthis reason represents the preferred method herein. In accordance withthis procedure, the aqueous solution of the iron and silver saltstogether with the phosphoric acid is obtained as previously noted.However, no precipitation is effected as the solution is subjected toheating with stirring in order to remove water. Heating is continueduntil the mass can be no longer stirred. The residue is then fragmentedand again heated at a moderately elevated temperature in the order ofabout 120° C. until completely dried. Thereupon the dried composite issized and calcined. Suitable calcination temperatures broadly range from400°-550° C. Applicable calcination periods range from 2-16 hours.

In the manner of either of these techniques, a supported catalyst can beprepared. For example, in the slurry method colloidal silica or anyother form thereof as well as other supports such as alumina, quartz,titanium dioxide, etc., can be added prior to removing the watercontent. Similarly, in the alternate method described, the precipitationof the metal phosphates can be accomplished in the presence of suspendedparticulates of the intended support.

The catalyst compositions of this invention can be employed in afluidized, stirred tank reactor, or fixed-bed type reactor. Because ofthe convenience associated with the use of a fixed-bed reactor in asmall scale operation, such a reactor will be exemplified herein. In thepreferred mode of operation the feed to the reactor comprises apre-heated gaseous mixture of the substrate, molecular oxygen, steam andinert diluent gas. A pre-heat temperature in the range of about 300° to350° C. is customarily observed. A broad range of applicable reactiontemperatures is from 300°-500° C. but more generally a temperature offrom 375° to 425° C. provides for optimum processing.

The mole ratio of molecular oxygen to substrate is from 0.5 to 1.5 andmore preferably from 0.7 to 0.75 in the case where the substrate isisobutyric acid per se. While steam is not necessary for the purpose ofeffecting the reaction, the presence thereof in the feed is particularlydesirable insofar as it acts as a heat sink and in such a capacityserves to minimize combustion of the substrate. An applicable mole ratioof water to the substrate in the feed is from about 8-20. Optimum ratiois more in the order of from 12 to 15.

Another important parameter resides in the concentration of thesubstrate in the feed. Expressed in terms of mole percents, theconcentration of the contemplated substrates ranges broadly from 0.1-20.As is common in reactions of this type, yield of the desired product isan inverse function of the concentration. From the standpoint ofachieving a reasonable through-put combined with an acceptable yield,the concentration of the substrate in the feed is from about 3-6 molepercent. Concentration is controlled by the amount of the inert gaspresent in the feed stream. The preferred diluent gas is nitrogenalthough other gases such as carbon dioxide, helium, argon, and the likeare suitable. Of course if the desired concentration of substratepermits, air represents a suitable diluted oxidant.

Another important parameter is that of contact time. Contact or reactiontime is defined as the catalyst volume divided by the volume of gas feedper second at reaction temperature. The catalyst volume is the bulkvolume occupied by the catalyst in the reactor. The term catalyst inthis sense not only includes the modified iron phosphate itself but alsothe support if present. Accordingly, applicable reaction times rangefrom 0.05-3.0 seconds and more generally in the order of from 0.1-1.0second. The reaction is carried out preferably at atmospheric pressurealthough a higher pressure up to about 10 atmospheres is applicable.

EXAMPLE I

The purpose of this example is to illustrate the hereinabove describedslurry method for preparing a supported catalyst useful in the practiceof this invention. Iron nitrate nona-hydrate in the amount of 103.21 g.along with 7.47 g. of silver nitrate were dissolved in 200 ml. ofdistilled water. Concentrated phosphoric acid in the amount of 35.8 g.together with 30.0 cc. of silica gel containing 20% SiO₂ were added tothe solution of the metal salts. The solution was then stirred at 85° C.until the bulk of the water had been evaporated. The resultant paste wasfurther dried at 120° C. until in condition to be fragmented whereupondrying was continued for 12 hours at 150° C. The dried mixture wascalcined at 450° C. for 16 hours and for an additional 2 hours at 520°C. The gram-atom empirical formula of the calcined mixed phosphates ofiron and silver follows: FeP₁.44 Ag₀.176 O_(x).

EXAMPLE II

In the like manner as in Example I a supported catalyst was prepared inwhich 30% of the catalyst system consisted of silica and titaniumdioxide as the support. The silica and titanium dioxide were present ina ratio of 1 to 2, respectively. The gram-atom empirical formula of themixed phosphates of iron and silver follows: FeP₁.45 Ag₀.2 O_(x).

EXAMPLE III

A further catalyst system was prepared as in Example I wherein nosupport was employed. The gram-atom empirical formula of the resultantmixed phosphates of iron and silver follows: FeP₁.44 Ag₀.16 O_(x).

EXAMPLE IV

This example illustrates the use of the catalyst compositions of theforegoing examples in effecting the oxidative dehydrogenation ofisobutyric acid as well as illustrating the like conversion of methylisobutyrate. The reactor and the general manner of conducting thereaction were the same for each of the enumerated runs. The procedureobserved consisted of feeding a pre-heated mixture of the applicablesubstrate, oxygen, nitrogen and steam through a stainless steel tube of1/2" OD (3/8" ID) and approximately 18" in length containing the testcatalyst as a 15 cc. packed bed maintained at the reaction temperatureutilized in the particular run.

The pre-heater consisted of a length of stainless steel tube similar tothe reactor but packed with glass beads. Any carbon dioxide formed inthe course of reaction was absorbed in an Ascarite tube protected by acalcium sulfate absorber for any uncondensed water. The condensedorganic product was separated from the water, collected and analyzed bythe internal standard method of gas chromotography.

Other pertinent processing conditions observed for the individual runsas well as the identity of the catalyst used therein are set forth inTable 1 presented hereinbelow. The results obtained in terms ofselectivity and conversion are likewise given in said table. Conversionrepresents the mole ratio of substrate consumed to that charged to thereactor. Selectivity to methacrylic acid in Run Nos. 1-4 represents themole ratio of methacrylic acid found in the effluent to that of IBAconsumed in the reaction. In Run No. 5 the converted product consistedof methacrylic acid (MAA) and methyl methacrylate (MMA) as accordinglyidentified.

                                      TABLE 1                                     __________________________________________________________________________                             CHARGE TO REACTOR                                                                          Substrate                                                                          Water                                                                              Total                                             Reaction    Substrate                                                                           (gm/ (gm/ Gas  Con-                                     Temp.                                                                             Time O.sub.2 /Substrate                                                                   Conc. hr @ hr @ Feed version                                                                           Selectivity          Run No.                                                                            Substrate                                                                           Catalyst                                                                           (°C.)                                                                      (Sec.)                                                                             (Mole Ratio)                                                                         (Mole %)                                                                            RT)  RT)  (l/hr)                                                                             (%) (%)                  __________________________________________________________________________    1    isobutyric                                                                          Ex. II                                                                             420 0.54 0.5    5     8.4  20   41.473                                                                             80  60                        acid                                                                     2    isobutyric                                                                          Ex. II                                                                             410 0.56  0.68  5     8.4  20   41.593                                                                             90  70                        acid                                                                     3    isobutyric                                                                          Ex. III                                                                            425 0.53 0.5    5     8.4  20   41.473                                                                             74  74                        acid                                                                     4    isobutyric                                                                          Ex. I                                                                              385 0.56  0.73  3.2   4.8  8.8  41.253                                                                             96  66                        acid                                                                     5    methyl                                                                              Ex. III                                                                            425 0.99 1.0    4.3   4.5  --   21.400                                                                             48  56(MMA)                   isobutyrate                                         -7(MAA)              __________________________________________________________________________

We claim:
 1. In a process for the catalytic conversion of isobutyricacid or a lower alkyl ester thereof to the correspondingα,β-ethylenically unsaturated derivative via the oxydehydrogenationreaction wherein an iron phosphate catalyst is contacted with a gaseousfeed stream containing said acid or ester substrate and oxygen at atemperature between about 300° and 500° C., the improvement of effectingsaid oxydehydrogenation reaction in the presence of a modified ironphosphate catalyst having the gram-atom empirical formula FeP₁₋₂Ag₀.01-1 O_(x) in which x represents the number of oxygen atoms bound tothe other elements in their respective states of oxidation in thecatalyst.
 2. The process in accordance with claim 1 wherein thesubstrate is isobutyric acid.
 3. The process in accordance with claim 1wherein the substrate is methyl isobutyrate.
 4. The process inaccordance with claim 2 or 3 wherein the modified iron phosphatecatalyst has the gram-atom formula FeP₁.4 Ag₀.16 O_(x).